Flame retardants that work via the mechanism of intumescence usually do not contain halogens. The flame-retardant mechanism of intumescence has been reviewed. (For a review of intumescence in coatings and polymers: Don G. Brady, C. Wayne Moberly, John R. Norell, and Harold C. Walters, J. Fire Retardant Chemistry, 4, p150(1977)). The intumescent flame retardant mechanism requires an inorganic acid source, a carbon source such as a polyhydric material like dipentaerythritol, and a blowing agent, which is often an amine like urea or melamine. Optionally, a halogen containing compound can be added for better activity. For coatings, the flame retardant includes the following types of compounds: a mineral acid salt such as sodium phosphate or practically water insoluble ammonia polyphosphate, a polyol such as starch, pentaerythritol, or dipentaerythritol, and a blowing agent such as melamine. The standard theory is that in a fire, the heat causes the mineral acid salt to decompose to form an acid, the acid dehydrates the polyol to form char, and the blowing agent decomposes to gaseous products. The result is char and gas that forms a foam that is much thicker than the original article or coating containing these flame retardants. A sequence of events with respect to formation of acid, dehydration of polyol, and release of gas must occur in the correct order and time sequence for the gas and char to form a protective foam. Different polymers may require different ingredients or amounts of ingredients to achieve similar levels of flame retardation. It is believed that the polymer and the flame retardant must have similar decomposition temperatures. Thus, different mineral acid salts, polyols, or blowing agents are used in different applications and there is no universal recipe.
Therefore, a need exists for a single compound that performed all the tasks of the mineral acid salt, the polyol, and the blowing agent and be generally applicable to a wide variety of polymers. Intumescence can be difficult to achieve in practice. It is often difficult for three or more ingredients to be well mixed in applications such as flame retarding a polymer. Good mixing of three ingredients in coating applications can be difficult if the ratio of solids content to solvent is very high. It is much more difficult to flame retard a polymer with three ingredients, because the above intumescence agents are added to the polymer melt. Relatively high viscosity of the polymer melt prevents easy mixing of flame retardants to obtain a homogeneous mix and good performance. Mixing a melted polymer for a long time to obtain a good dispersion of the flame retardants is unacceptable as the polymer can degrade if held above melt temperature too long. The flame resistance of polyolefins such as polypropylene can be improved by adding melamine pyrophosphate (MPP) and dipentaerythritol. (as taught in U.S. Pat. No. 3,936,416, 1976). This patent teaches that multiple components need be mixed into the polypropylene for good flame retardant performance via intumescence, as melamine pyrophosphate by itself requires too high a loading. Flame retardant performance will be dependent on uniformity of mixing of the components melamine pyrophosphate and dipentaerythritol into polypropylene. A single compound flame retardant would be easier from a mixing standpoint as maintaining the flame retardant in close proximity and balance throughout would not be as crucial. For plastics in general, it is difficult to disperse the ingredients as each ingredient may disperse differently or even agglomerate in the polymer melt.
Ethylene diamine phosphate (EDAP), which has some intumescence, is an excellent flame retardant for olefins such as polypropylene. Unfortunately, commercial extruders process polypropylene at about 235° C. which is too high a temperature to safely use EDAP without extensive ventilation to capture ethylene diamine that is released. Thus, it would be most desirable to make flame retardants that are more stable than EDAP and which would be good flame retardants for polymers such as polypropylene. Flame retardants such as EDAP require special conditions on commercial extruders to be used without decomposition. A flame retardant that is stable under standard processing conditions is highly desirable.
A single compound that intumeses is discussed in PCT/US01/09514. The most preferred examples are given as methylol melamine salts of polyphosphoric acid or pyrophosphoric acid. Such compounds gas during extrusion and molding leading to undesirable mechanical properties. These compounds do not partially dissolve into the polymer during compounding as these compounds are not resinous in appearance or behavior. The best practice compounds are not as effective in flame retarding olefin polymers as the compounds described herein. Examples 6 and 7 of PCT/US01/09514 describe preparation of ethylene diamine reaction product with pyrophosphoric acid. The preparations used too much ethylene diamine and the preparations contained pyrophosphoric acid contaminated with substantial sodium resulting in the formation of white particulate that could be filtered and dried, unlike the flame retardants of the invention. The procedures in PCT/US01/09514 utilize drying and filtering, not evaporation.
The flame retardants of the invention address the need for such a more temperature stable flame retardant agent for olefins and other polymers which does not gas undesirably during processing, and which can be machine processed at temperatures 235° C. or even higher.